Over the past few decades, there has been a huge interest in materials that can heal themselves because self-healing systems can increase materials lifetime, reduce replacement costs and improve the safety of the products. Self-healing materials can be classified broadly into three groups: capsule based, vascular, and intrinsic.
In the capsule based self-healing materials, upon damage-induced cracking, the microcapsules are ruptured by the propagating crack fronts resulting in release of the healing agent into the cracks by capillary action. Subsequent chemical reaction between the healing agent and the embedded catalyst heals the material and prevents further growth of the crack.
Microencapsulation has been one of the most efficient and broadly used approaches in self-healing materials development.
Microencapsulation is a process in which small droplets of liquid or particles are surrounded with a thin film. A microcapsule is a small sphere with a uniform wall around it. The material inside the microcapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called a shell, coating, or membrane. The configuration of the core can be a spherical or irregular particle, liquid-phase suspended solid, solid matrix, dispersed solid and aggregates of solids or liquid forms.
Microcapsules can be classified to three basic categories according to their morphology, namely, mononuclear, polynuclear, and matrix types. Mononuclear (core-shell) microcapsules contain a shell around the core, while pollynuclear capsules have many cores enclosed within a shell. In the matrix encapsulation, the core material is distributed homogeneously into the shell material. In addition to the aforementioned three basic morphologies, microcapsules can also be in a form of a mononuclear within multiple shells, or form clusters of microcapsules.
There are several methods of microencapsulation, namely, solvent diffusion method, spray drying method, spray congealing method, coacervation, phase separation method, polymerization, and emulsification/solvent evaporation method. However, emulsification/solvent evaporation techniques may be more useful compared to other methods. A suitable particle size control in the range of nanometers to micrometers can be achieved using this method, but there is a need for careful selection of encapsulation materials and various conditions in order to achieve high encapsulation efficiency and a low residual solvent content.
Several process variables are identified in the prior art, which could affect the formulation of microspheres produced by emulsification/solvent evaporation method; variables such as solvent type, solvent volume, active material to polymer ratio, rate of solvent removal, effect of internal aqueous phase volume in case of using the method of solvent evaporation followed by multiple emulsion, the effect of buffer or salt addition to the internal or external phase which can affect the size of microspheres and also the release pattern of the active material from the microspheres.
As is reported in the prior art, water-oil-water multiple emulsion system for microsphere preparation overcomes the problem of low encapsulation efficiency of water soluble active materials in the conventional water/oil emulsion solvent evaporation method.
It should be understood by a person skilled in the art that the emulsification is the first step of the emulsification/solvent evaporation method and has been extensively investigated in prior art. On the contrary the second step, the solvent transport out of the emulsion droplets, which determines the particle morphology and has a great influence on the microparticles encapsulation and release behavior has been scarcely studied. Usually, the solvent is highly volatile, which makes the solvent elimination process very fast and thus, difficult to observe.
The most desirable approach is to prepare capsules containing epoxy and its hardener because they are the exact materials used to prepare coatings and incorporation of these capsules in the epoxy matrix can build a repair system which is compatible with the host matrix. Several methods for encapsulation of epoxy resin are known from prior art. However, preparation of capsules containing a liquid amine is very difficult and only several attempts have been made to encapsulate reactive amine, because amine is soluble in both water and organic solvents.
A method for preparation of microcapsules containing diethylenetriamine (DETA) by interfacial polymerization is disclosed in prior art. But, the core content of the resultant capsules has high viscosity and it limits the healing capability of the system. In another method disclosed in prior art, hollow capsules are prepared and then they are filled with the reactive amine, but the yields for this encapsulation procedure ranged from 9.6% to 17.7%, and as is understood by a person skilled in the art, this is not a desirable core content for self-healing applications.
In another attempt disclosed in prior art, microcapsules containing reactive amine are prepared by multi emulsion system. However, this method had several flaws, such as, the large size distribution of microcapsules, very low core content (about 20%) and very long reaction time.
Two methods are disclosed in the prior art for preparation of microcapsules containing a hardener. However, it should be known that the prepared microcapsules in general have several disadvantages: they cannot be dispersed easily in the polymeric matrix; they would be visible in coatings and homogenous coating could not be obtained and microcapsules cannot be used in thin coatings in which the thickness of the coating is less than 1 micrometer.
The use of microcapsules containing solvent in self-healing composites is disclosed in the prior art. In this method, microcapsules are broken upon fracture, and the solvent is released in the crack. The big disadvantage of this method is the difference between the solvent and the matrix material.
The method of preparation of multiple emulsions by mixing a first emulsion in a second aqueous phase to form polymer beads is also disclosed in the prior art. This method produces porous polymer particles having a large size distribution with little control over the porosity, which, is not suitable for self-healing purposes.
Therefore, it is necessity to develop a process by which an amine can be encapsulated simply with high core content because it would reduce manufacturing cost and time consumption for nanocapsules production. There is also a need to provide flexible processes so that a wide range of nanocapsules containing different healing agents can be easily prepared.